This invention relates to a control method and device for reproducing a halftone dot cut away along an outline of a reproduction picture, for use in a picture reproducing machine such as a color scanner or a color facsimile, wherein a halftone picture is reproduced by scanning.
There are already known many methods for reproducing a halftone picture by means of a picture reproducing machine such as a color scanner for plate-making or a color facsimile according to picture signals picked up by scanning an original picture. For example, Japanese Patent Laying-Open Specification No. 54-79701 (Japanese Patent Application No. 52-145683) has been filed by the same applicant assignee as the present invention, and it discloses a machine for reproducing a halftone picture by scanning, as shown in FIG. 1.
In this application, as shown in FIG. 1, a light beam emitted by a light source 1 such as a laser tube is passed along its light axis through the first acoustooptical deflector element 2, a V-shaped aperture of an aperture plate 3, the second acoustooptical deflector element 4, and a focusing lens 5 in order to obtain a minute line image w. The deflector element used in this embodiment, as is already well-known, is of a type which makes use of the fact that elastic deformation in a crystal caused by ultrasonic waves can function as a diffraction grating. This deflector element is capable of suitably changing the pitch of the diffraction grating by controlling the frequency of the ultrasonic waves supplied to the crystal, so as to deflect the light beam at an adjustable diffraction angle. The deflector element is capable of oscillating at deflection frequencies and of controlling the amplitude of deflection angle as desired.
Hence, the first deflector element 2 diffracts the light beam vertically, i.e. in the Z direction, at a desired angle depending on the ultrasonic frequency supplied thereto so that the height of the light beam projected onto the aperture may be varied in order to vary the width of the light beam therethrough. The second deflector element 4 diffracts the light beam horizontally, i.e. in the Y direction, at a desired angle depending on the ultrasonic frequency supplied thereto so that the position of the light beam through the aperture may be shifted in the direction of its width. Thus, the width and the position of the line image w of the light beam can be varied by controlling the ultrasonic frequencies supplied to the two deflector elements 2 and 4, and by using this line image w the halftone picture can be reproduced onto a photosensitive material mounted to a recording cylinder, as shown in FIG. 2.
There are shown three halftone dot patterns to be recorded, having screen angles 45.degree., 0.degree. and 15.degree. with respect to a main scanning line S in FIGS. 2I, 2II and 2III. If left and right lengths which are obtained by dividing the width of the halftone dot into left and right parts by the scanning line S, are L and R, the entire length W of the line image w is expressed as follows. EQU W=L+R
In this embodiment described above, the pitch of the scanning lines of the color scanner for plate-making is smaller than that of the screen lines of a usual halftone screen, and thus several scanning lines cross one halftone dot area. Hence, one halftone dot is recorded by one scanning operation of one of the several scanning lines.
In FIGS. 2I or 2II the scanning line S which divides the halftone dot area into two equal parts is selected and along the scanning line selected the halftone dot is recorded. In FIG. 2III the scanning line S which passes through one corner of the halftone dot area is selected for recording the halftone dot.
In the embodiment shown in FIG. 2III, a diagonal line S' of the halftone dot area passes the one corner the scanning line S passes, and divides the halftone dot area into the two equal parts. If left and right lengths which are obtained by dividing the width of the halftone dot area in the direction perpendicular to the scanning line S into two left and right parts by the diagonal line S', are .alpha. and .beta., and the distance between the scanning line S and the diagonal line S' in the direction perpendicular to the scanning line S is .sigma., the lengths L and R are expressed in the following formulae. EQU L=.alpha.-.sigma. EQU R=.beta.+.sigma.
These formulae are satisfied in the embodiments shown in FIGS. 2I or 2II, wherein .sigma.=0.
Consequently, the lengths L and R are obtained from the lengths .alpha. and .beta. according to the above formulae and are then summed to give the entire length W. Then, the deflection control of the light beam by means of the first deflector element is so carried out that the length of the line image w may be the resulting entire length W.
A distance p between the scanning line S and the center of the length W of the line image w is given in the following formula. EQU p=(R-L)/2
In the embodiments shown in FIGS. 2I and 2II, R equals L, and accordingly p equals nought. Hence, it is unnecessary to diffract the light beam by the second deflector element. However, in the embodiment shown in FIG. 2III, R does not equal L and the light beam is to be deflected by the second deflector element. This deflection operation is controlled so that the center of the line image w may be moved along a one-dotted line C shown in FIG. 2III.
In this case, of course, the halftone dot area may be varied depending on the density of the picture signals. This can be carried out by changing the width and the position of the line image by controlling the two deflector elements.
According to this method, the halftone dots are always recorded in their entire forms. However, in practice, the halftone dots should often be cut away along the outline of the reproduction picture. In such a case, if no halftone dots are cut away along the outline, a faithful reproduction picture cannot be obtained.